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486                              CAPE   JUMPING   HARE                             CHAP.
Alactaga is much like JEitchoreutes; It lias five toes, a
cylindrical tufted tail, the hairs at the end distichous, smooth
incisors, and a premolar present in the upper jaw. It also
differs from EucJwreutes by the much smaller auditory bulla as well
as in the fact that the infra-orbital foramen lias no separate
passage for the nerve, which passage is to be distinguished in
both Dipus and JS'ucTioreutes. The best-known species is the
Siberian Jumping Babbit, -4. jaculus. Beneath the ends of the
three main toes of the feet are remarkable fan-shaped pads. In
A. decnmana the body and tail measure 7 and 10 laches re-
spectively, the ears 2 inches. PlatyceTcomys^ a fourth genus of
the family, is much less known and is to be differentiated from the
last three genera by the fact that it lias no premolars at all, the
grinding tooth formula being thus -|. The tail too is flattened
and " lancet shaped." Ib extends from Siberia to Nubia, and
thus just enters the Ethiopian region.
The above are the more typical Jerboas. There remain
several forms which are not at all Jerboa-like in their way of
life, but are nevertheless, on anatomical grounds, placed with
them. Zapus, an American genus, with the exception of one
Palaearctic species, is transitional in that its hind-legs are
rather long, but there is not so much difference between them as
in the typical Dipodidae. Sminthus is at the opposite extreme
to Dipus. Its feet are short and of equal length; it climbs in
trees, and may perhaps be looked upon as nearest of all Dipodidae
to the ancestral form of the group.
Fam. 8. Pedetidae.—The genus Pedetes contains but one
species, P. caffer, the Cape Jumping Hare. The animal suggests
a large Jerboa in appearance on account of its jumping habits,
the long hind-limbs, and the long tail The length of a fair-
sized example is some 17 inches, with a tail of the same length.
The eyes and ears are large. The hands are five-fingered and
the feet only four-toed, the hallux being of course the absent
digit. In the skeleton it is interesting to note that the second
and third cervical vertebrae are so close together that there can
be no free movement; interesting because in Dipus the cervicals
are actually ankylosed. The dorsal vertebrae are twelve. The
small intestine is long, measuring 7 feet 4 inches, while the
caecum is short, being only 8 inches long. The large intestine
is 3 feet 10 inp.hAs lr>«<* TK^ r*«ii -ui«,u— --------- ^ •«